Electric Fields ____ 1. Two point charges are 4 cm apart. They are moved to a new separation of 2 cm. By what factor does the resulting mutual force between them change? a. 1/2 c. 1/4 b. 2 d. 4 ____ 2. If the distance between two point charges is tripled, the mutual force between them will be changed by what factor? a. 9.0 c. 0.33 b. 3.0 d. 1/9 ____ 3. If the size of the charge value is tripled for both of two point charges maintained at a constant separation, the mutual force between them will be changed by what factor? a. 9.0 c. 0.33 b. 3.0 d. 1/9 ____ 4. The electric field of a point charge has an inverse ________ behavior. a. r1/2 c. r2 b. r d. r3 ____ 5. A thin uncharged conducting spherical shell has a charge q carefully placed at its center through a small hole in the shell. The charge q does not touch the shell. What is the charge on the shell? a. q c. 2q b. q d. 0 ____ 6. The combination of two separated point charges of opposite sign but equal magnitude is called an electric: a. monopole. c. quadrapole. b. dipole. d. magnapole. ____ 7. A charge, +Q, is placed inside a balloon and the balloon is inflated. As the radius of the balloon r increases the number of field lines going through the surface of the balloon: a. increases proportional to r2. c. stays the same. b. increases proportional to r. d. decreases as 1/r. ____ 8. A spherical volume of space has an electric field of intensity 100 N/C directed radially outward from it surface of radius 0.600 m. What is the net charge enclosed within this surface? a. 6.8 nC c. –6.8 nC b. 4.0 nC d. –4.0 nC ____ 9. A metallic object holds a charge of 3.8 106 C. What total number of electrons does this represent? (e = 1.6 1019 C is the magnitude of the electronic charge.) a. 4.2 1014 c. 2.4 1013 b. 6.1 1013 d. 1.6 1014 ____ 10. Two point charges, separated by 1.5 cm, have charge values of +2.0 and 4.0 µC, respectively. What is the value of the mutual force between them? (ke = 8.99 109 Nm2/C2) a. 320 N c. 8.0 1012 N b. 3.6 108 N d. 3.1 103 N 1 ____ 11. In a thundercloud there may be an electric charge of +40 C near the top of the cloud and 40 C near the bottom of the cloud. These charges are separated by about 2.0 km. What is the electric force between these two sets of charges? (ke = 8.99 109 N·m2/C2) a. 3.6 104 N c. 3.6 106 N 5 b. 3.6 10 N d. 3.6 107 N ____ 12. An electron is sent at high speed toward a gold nucleus (charge +79e). What is the electrical force acting on the electron when it is 3.0 1014 m away from the gold nucleus? (e = 1.6 1019 C, ke = 8.99 109 N·m2/C2) a. 20 N c. 2.0 104 N b. 0.25 N d. 2.1 106 N ____ 13. Two electrons are separated by one cm. What is the ratio of the electric force to the gravitational force between them? (me = 9.11 1031 kg, ke = 8.99 109 Nm2/C2, G = 6.67 10-11 Nm2/kg2, and e = 1.6 1019 C) a. 2.3 102 c. 3.1 1022 20 b. 1.3 10 d. 4.2 1042 ____ 14. Two equal charges, each Q, are separated by some distance. What third charge would need to be placed half way between the two charges so that the net force on each charge would be zero? a. Q c. Q/4 b. Q/2 d. Q/8 ____ 15. A 6.0 C charge is placed at the origin and a second charge is placed on the x-axis at x = 0.30 m. If the resulting force on the second charge is 5.4 N in the positive x-direction, what is the value of its charge? a. 9.0 C c. –9.0 C b. 9.0 nC d. –9.0 nC ____ 16. A 6.00 C charge is placed at the origin and a second charge is placed on the x-axis at x = 0.300 m. If the resulting force on the second charge is 6.40 N in the positive x-direction, what is the force on the charge at the origin? a. 6.40 N in the positive x-direction c. 0 N b. 6.40 N in the negative x-direction d. not able to be determined until the second charge is known ____ 17. Two point charges each have a value of 30.0 mC and are separated by a distance of 4.00 cm. What is the electric field midway between the two charges? (ke = 8.99 109 N·m2/C2) a. 40.5 107 N/C c. 10.1 107 N/C 7 b. 20.3 10 N/C d. zero ____ 18. Two point charges are separated by 10.0 cm and have charges of +2.00 µC and 2.00 µC, respectively. What is the electric field at a point midway between the two charges? (ke = 8.99 109 N·m2/C2) a. 28.8 106 N/C c. 7.19 106 N/C 6 b. 14.4 10 N/C d. zero ____ 19. An electron with a charge value of 1.6 1019 C is moving in the presence of an electric field of 400 N/C. What force does the electron experience? c. 6.4 1017 N a. 2.3 1022 N 21 b. 1.9 10 N d. 4.9 1017 N 2 ____ 20. The average distance of the electron from the proton in the hydrogen atom is 0.51 1010 m. What is the electric field from the proton’s charge at the location of the electron? (ke = 8.99 109 Nm2/C2, e = 1.6 1019 C) a. 5.5 1011 N/C c. 3.2 102 N/C 6 b. 1.0 10 N/C d. 8.8 108 N/C ____ 21. A proton initially moves left to right long the x axis at a speed of 2.00 103 m/s. It moves into an electric field, which points in the negative x direction, and travels a distance of 0.200 m before coming to rest. What acceleration magnitude does the proton experience? c. 9.33 109 m/s2 a. 6.67 103 m/s2 b. 1.00 107 m/s2 d. 2.67 1011m/s2 ____ 22. A proton initially moves left to right long the x axis at a speed of 2.00 103 m/s. It moves into an electric field, which points in the negative x direction, and travels a distance of 0.200 m before coming to rest. If the proton's mass and charge are 1.67 1027 kg and 1.60 1019 C respectively, what is the magnitude of the electric field? a. 28.3 N/C c. 0.104 N/C b. 13.9 N/C d. 0.038 N/C ____ 23. Two charges, +Q and Q, are located two meters apart and there is a point along the line that is equidistant from the two charges as indicated. Which vector best represents the direction of the electric field at that point? a. b. Vector EA Vector EB c. d. Vector EC The electric field at that point is zero. ____ 24. A charge of +2 C is at the origin. When charge Q is placed at 2 m along the positive x axis, the electric field at 2 m along the negative x axis becomes zero. What is the value of Q? a. b. 3 C 6 C c. d. 7 C 8 C ____ 25. An electron with a speed of 2.0 106 m/s moves into a uniform electric field of 500 N/C that is parallel to the electron’s motion. How long does it take to bring the electron to rest? (me = 9.11 1031 kg, e = 1.6 1019 C) a. 2.3 108 s c. 1.2 107 s 8 b. 3.5 10 s d. 2.3 106 s 3 ____ 26. In x-ray machines, electrons are subjected to electric fields as great as 6.0 105 N/C. Find an electron’s acceleration in this field. (me = 9.11 1031 kg, e = 1.6 1019 C) a. 1.1 1017 m/s2 c. 4.6 1010 m/s2 b. 5.4 1013 m/s2 d. 3.6 108 m/s2 ____ 27. An airplane is flying through a thundercloud at a height of 2 000 m. (This is a very dangerous thing to do because of updrafts, turbulence, and the possibility of electric discharge.) If there is a charge concentration of +40 C at height 3 000 m within the cloud and 40 C at height 1 000 m, what is the magnitude of the electric field E at the aircraft? (ke = 8.99 109 N·m2/C2) a. 90 000 N/C c. 360 000 N/C b. 180 000 N/C d. 720 000 N/C ____ 28. Electrons in a particle beam each have a kinetic energy of 3.2 1017 J. What is the magnitude of the electric field that will stop these electrons in a distance of 0.1 m? (e = 1.6 1019 C) a. 200 N/C c. 2 000 N/C b. 1 000 N/C d. 4 000 N/C ____ 29. The electric field in a cathode ray tube is supposed to accelerate electrons from 0 to 1.60 107 m/s in a distance of 2.00 cm. What electric field is required? (me = 9.11 1031 kg and e = 1.60 1019 C) a. 9 110 N/C c. 36 400 N/C b. 18 200 N/C d. 72 800 N/C ____ 30. Two point charges, separated by 1.5 cm, have charge values of +2.0 and 4.0 µC, respectively. Suppose we determine that 10 field lines radiate out from the +2.0-µC charge. If so, what might be inferred about the 4.0-C charge with respect to field lines? a. 20 radiate out c. 20 radiate in b. 5 radiate out d. 10 radiate in ____ 31. Charge A has 10 electric field lines coming out, Charge B has 20 lines coming out, and Charge C has 30 lines coming in. Which pair of these charges will have the largest force between them if placed one cm apart? a. A and B c. C and A b. B and C d. More information is needed. ____ 32. Q1 has 50 electric field lines radiating outward and Q 2 has 100 field lines converging inward. What is the ratio Q1/Q2? a. 2 c. 1/2 b. 2 d. 1/2 ____ 33. An initially uncharged hollow metallic sphere with radius of 5 cm has a small object with a charge of +10 µC carefully placed at the center of the sphere through a hole in the latter’s surface. With the charge in place, what charge is now present on the outside surface of the sphere? a. zero c. +4 000 µC b. 10 µC d. +10 µC ____ 34. An initially uncharged hollow metallic sphere with radius of 5 cm has a small object with a charge of +10 µC carefully placed at the center of the sphere through a hole in the latter’s surface. What charge resides inner surface of the sphere? a. 4 000 µC c. +10 µC b. 10 µC d. zero 4 ____ 35. We have an initially uncharged hollow metallic sphere with radius of 5.0 cm. I place a small object with a charge of +10 µC at the center of the sphere through a hole in the surface. Find the electric field present at a point 10 cm from the sphere’s center. (ke = 8.99 109 Nm2/C2) a. 1.1 106 N/C c. 9.0 106 N/C b. 2.3 106 N/C d. 36 106 N/C ____ 36. Two identical spheres each carry a charge of 40.0 µC. The spheres are separated by a distance of 1.00 m. What is the electric force between the spheres? (ke = 8.99 109 N·m2/C2) a. 28.8 N (repulsive) c. 7.19 N (attractive) b. 14.4 N (repulsive) d. 43.2 N (attractive) ____ 37. A ping-pong ball covered with a conducting graphite coating has a mass of 5.0 103 kg and a charge of 4.0 µC. What electric field directed upward will exactly balance the weight of the ball? (g = 9.8 m/s2) a. 8.2 102 N/C c. 2.0 107 N/C 4 b. 1.2 10 N/C d. 5.1 106 N/C ____ 38. In Millikan’s oil drop experiment, if the electric field between the plates was of just the right magnitude, it would exactly balance the weight of the drop. Suppose a tiny spherical oil droplet of radius 1.6 104 cm carries a charge equivalent to one electron. What electric field is required to balance the weight? (The density of oil is 0.85 g/cm3, e = 1.6 1019 C.) a. 1.1 105 N/C c. 4.5 105 N/C 5 b. 2.2 10 N/C d. 8.9 105 N/C ____ 39. A charge Q accumulates on the hollow metallic dome, of radius R, of a Van de Graaff generator. A probe measures the electric field strength at various points outside the sphere surface. If the probe is initially at a distance 3R from the sphere's center and then is moved to 4R, by what factor will the electric field reading change? a. (4/3)2 c. 3/4 b. 4/3 d. (3/4)2 ____ 40. A charge Q accumulates on the hollow metallic dome, of radius R, of a Van de Graaff generator. A probe measures the electric field strength at various points outside the sphere surface. By what factor will the electric field value at the 2R distance be changed if the charge value were increased to (4/3)Q? a. (4/3)2 c. 3/4 b. 4/3 d. (3/4)2 ____ 41. A closed surface contains the following point charges: 6 C, 4 C, –2 C, –4 C. The electric flux coming out of the surface is: c. 4 C/o. a. 16 C/o. b. –16 C/o. d. –4 C/o. ____ 42. Object A has a charge q on it, object B has a charge q on it, and object C has a charge 2q on it. These charges are arranged, one each, at the vertices of an equilateral triangle. Which charge has the greatest magnitude electric force on it? a. A c. C b. B d. All have equal magnitude forces on them. 5 ____ 43. Three equal positive charges are placed on the x-axis, one at the origin, one at x = 2 m, and the third at x = 4 m. Of the following points, which has the greatest magnitude electric field? a. x = 1 m c. x = 5 m b. x = 3 m d. The electric field has the same magnitude at all three positions. ____ 44. A spherical surface surrounds a point charge it its center. If the charge is doubled and if the radius of the surface is also doubled, what happens to the electric flux E out of the surface and the magnitude E of the electric field at the surface as a result of these doublings? a. E and E do not change. c. E increases and E decreases. b. E increases and E remains the same. d. E increases and E increases. 6 ID: A Electric Fields Answer Section MULTIPLE CHOICE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. D D A C D B C B C A C A D C A B D B C A B C A D A A D C C C B D D B C B B D D B 1 ID: A 41. 42. 43. 44. C C C C 2